The ARVN tended to use a mix of canned rations and locally purchased and freshly cooked rations, the typical staples of a Vietnamese diet like rice, fish, fish sauce and various vegetables. To this end ARVN troops were supposed to be paid a small ration allowance to purchase fresh produce, this was withheld when supplied with the tinned, operational ration, so where possible the preference was purchase of fresh rations.
Sadly, supply tended to be insufficient and there are reports of ARVN units with cases of malnutrition. Often this was a direct result of graft in their supply system. The US did support the ARVN supply system supplying canned goods and dehydrated rice but apparently that often did not make its way down to soldiers in the field. Theft of food from the civilian population was at times a significant issue with ARVN units.
On occasion they would use US C rations to supplement their rations, often unofficial donations from US units operating in the area, so yes C rations are a possibility. That said, the Vietnamese palate didn’t much care for US tinned produce. Having eaten both Vietnamese food and tinned C Ration type meals extensively I can thoroughly understand their preferences. Vietnamese food is sublime.
So yes, you could add a C ration carton here or there but I’d add rice sacks and maybe bundles of fresh greens to balance it out. Even a chicken crate.